CBSE Class 9 Science Force and Laws of Motion MCQs Set D

Question: A body, whose momentum is constant, must have constant
a) Force
b) Velocity
c) Acceleration
d) All of these
Answer: b

Question: The inertia of a moving object depends on
a) Momentum of the object
b) Speed of the object
c) Mass of the object
d) Shape of the object
Answer: c

Question: If A and B are two objects with masses 6 kg and 34 kg respectively, then
a) A has more inertia than B
b) B has more inertia than A
c) A and B have same inertia
d) None of the two has inertia
Answer: b

Question: A man getting down a running bus, falls forward because
a) Due to inertia of rest, road is left behind and man reaches forward
b) Due to inertia of motion, upper part of body continues to move forward while feet come to rest as soon as they touch the road
c) He leans forward as a matter of habit
d) Combined effect of all factors (a), (b), and (c)
Answer: b

Question: A cannon after firing recoils due to
a) Conservation of energy
b) Backward thrust of gases produced
c) Newton’s third law of motion
d) Newton’s first law of motion
Answer: c

Question: If a body is moving along at constant speed, it may be assumed that
a) A net force is pushing it forward
b) The sum of only vertical forces is zero
c) The buoyant force is greater than gravity
d) The sum of all forces is zero
Answer: d

Question: Swimming is possible on account of
a) First law of motion
b) Second law of motion
c) Third law of motion
d) Newton’s law of gravitation
Answer: c

Question: The momentum of a body of given mass is proportional to its
a) Velocity
b) Size
c) Shape
d) All of these
Answer: a

Question: A body is accelerating in a straight line. The unbalanced force acts
a) In the direction of motion of the body
b) In a direction opposite to the motion
c) In a direction perpendicular to motion
d) None of these
Answer: a

Question: A cannon after firing recoils due to
a) Conservation of energy
b) Newton’s third law of motion
c) Newton’s first law of motion
d) None of these
Answer: b

Question: Identify the correct statement(s).
1. To accelerate the motion of an object, a balanced force is required.
2. Balanced forces do not change the state of rest or of motion of an object.
3. Balanced forces do not produce any acceleration, they can change the shape or size of the body.
(a) 1 and 2
(b) 2 and 3
(c) 1 and 3
(d) None of these
Answer: b

FILL IN THE BLANK

Question: Application of a force changes the ………. of an object.
Answer: velocity

Question: If there are several forces on an object, its acceleration depends on its mass and the ………. force.
Answer: net unbalanced.

Question: An object moving at constant speed is in a state of ……….
Answer: equilibrium

Question: ………. is equal to change in momentum.
Answer: impulse

Question: In any interaction between two or more isolated objects, the total ………. does not change.
Answer: momentum

Question: The SI unit of force is the …………
Answer: Newton

Question: The change in the velocity of an object is proportional to the ……… applied to it.
Answer: impulse

TRUE/FALSE

Question: A body can be in equilibrium under the action of three coplaner forces.
Answer: True

Question: It is easier to start motion in a lighter body than a heavier body.
Answer: True

Question: Action and reaction act on the same body.
Answer: False

Question: If particle is initially at rest then it moves in direction of net force.
Answer: True

Question: Particle is at rest, if force is zero.
Answer: False

Question: Action and reaction force acts on the same object.
Answer: False

ASSERTION AND REASON

Question: Assertion : A quick collision between two bodies is more violent than a slow collision, even when the initial and the final velocities are identical.
Reason : The rate of change of momentum determines the force.
Options:
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Both Assertion and Reason are false.
Answer: a

Question: According to law of inertia (Newton’s first law), when cloth is pulled from a table, the cloth come in state of motion but dishes remains stationary due to inertia.
Assertion : When a firefly hits a bus, each of them exerts the same force.
Reason : Firefly has more mass as compared to the windshield.
Options:
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Both Assertion and Reason are false.
Answer: c

Question: Assertion : Force required to accelerate a mass in two perpendicular directions is never same.
Reason : The presence of g will not influence the acceleration.
Options:
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Both Assertion and Reason are false.
Answer: c

Question: Assertion : If the net external force on the body is zero, then its acceleration is zero.
Reason : Acceleration does not depend on force.
Options:
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Both Assertion and Reason are false.
Answer: c

Question: Assertion : While walking on ice, one should take small steps to avoid slipping.
Reason : This is because smaller steps ensure smaller friction.
Options:
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Both Assertion and Reason are false.
Answer: a

Question: Assertion : A table cloth cannot be pulled from a table without dislodging the dishes.
Reason : Newton’s second law of motion gives definition of inertia.
Options:
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Both Assertion and Reason are false.
Answer: d

Question: Assertion : Change in momentum is impulse.
Reason : Impulse is the area between (F-t) graph and time axis.
Options:
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Both Assertion and Reason are false.
Answer: b

Question: Assertion : From Newton’s second law of motion, impulse is equal to change in momentum.
Reason : Impulse and momentum have different SI units.
Options:
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Both Assertion and Reason are false.
Answer: d

Question: Assertion : A quick collision between two bodies is more violent than a slow collision, even when the initial and the final velocities are identical.
Reason : Because the rate of change of momentum which determines the force is greater in the first case.
Options:
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Both Assertion and Reason are false.
Answer: a

Question: Assertion : A large number of concurrent forces acting at the same point of the object, then the object will be in equilibrium, if sum of all the forces is equal to zero.
Reason : Equilibrium of a particle in mechanics refers to the situation when the net external force on the particle is non-zero.
Options:
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Both Assertion and Reason are false.
Answer: c